In order to deal with the exponential increase in network traffic due to the prevalent use of the Internet and intranets in offices and homes, there exists a need for an optical path switching device (optical switch) which does not employ an electric signal, namely, an optical-to-optical direct switch. Known approaches of the device and method for switching optical paths such as optical fibers, optical waveguides, and light propagation paths may include a space division type in which optical paths are switched within an optical waveguide or between optical waveguides, a wavelength division multiplex type in which switching is performed by splitting a multiplexed light having a plurality of wavelengths into respective optical paths according to the wavelengths, a time division multiplex type in which optical paths of a time-division multiplexed light are switched periodically, and a free space type in which light propagation paths in an open space are spatially divided and compounded using mirrors, shutters, or the like. Each of these approaches may be multiplexed, and a combination of these approaches may be used.
A space division type optical switch is still under development, but a number of approaches have been proposed, including a type using a directional coupler, a type in which a copy of the optical signal is produced using an optical divider and a light is switched on and off by a gate element, and a type in which the refraction index of a waveguide is changed at a crossing or a crossing portion of a Y-branch so as to transmit or reflect the light propagated in the waveguide. A Mach-Zehnder interferometer type optical waveguide switch in which the refraction index of the waveguide is changed by a thermooptic effect caused by heating with an electric heater has recently been publicized as being at a state of development close to actual use. However, a switch of this type is disadvantageous not only in its slow response speed of approximately 1 millisecond, but also in that an electric signal must be used to operate the optical switch.
As free space type optical switches, development efforts are being made towards approaches such as a MEMS (micro electro mechanical system), an EARS (exciton absorption reflection switch), a multistage beam shifter type optical switch, a hologram type optical switch, and a liquid crystal switch. These switches have disadvantages in that they include mechanically moving parts and have polarization dependency. It is therefore considered that a free space type optical switch is not yet ready for actual use.
Much research is directed to development of all-optical optical elements and optical control methods which make use of changes in transmittance and refraction index generated by irradiating light on an optical element, to thereby directly use light to modulate light intensity and frequency.
For the purpose of developing a novel information processing technique employing all-optical optical elements and the like, the present inventors have been conducting research on optical control methods using an organic nanoparticle photothermal lens-forming element obtained by dispersing organic dye aggregates in a polymer matrix (disclosed in Takashi HIRAGA, Norio TANAKA, Kikuko HAYAMI, and Tetsuro MORIYA, “Production, Structure Evaluation, and Photophysical Properties of Dye Clusters and Aggregates”, Electrotechnical Laboratory Report, published by Electrotechnical Laboratory (Japan), Agency of Industrial Science and Technology, Ministry of International Trade and Industry, Vol. 59, No. 2, pages 29–49 (1994)). At the present, an element which uses a control light (633 nm) to modulate a signal light (780 nm) has been developed. In this element, the control light and the signal light are arranged to be incident coaxially and parfocally. The operation principle is such that absorption of the control light allows temporary formation of a thermal lens which refracts the signal light. This element achieves a high response speed of approximately 20 nanoseconds.
Japanese Patent Laid-Open Publications Nos. Hei 8-286220, Hei 8-320535, Hei 8-320536, Hei 9-329816, Hei 10-90733, Hei 10-90734, and Hei 10-148852 describe an optical control method in which a control light is irradiated on an optical element having an optically responsive composition, so as to reversibly change the transmittance and/or refraction index of a signal light within a wavelength band differing from that of the control light, thereby modulating intensity and/or luminous flux density of the signal light transmitted through the optical element. According to this method, the control light and the signal light are each converged and irradiated on the optical element, while the optical paths of the control light and the signal light are adjusted such that the control light and the signal light overlap in their regions (beam waists) in the vicinity of the respective foci having the highest photon density.
Japanese Patent Laid-Open Publication No. Hei 10-148853 discloses an optical control method in which an optical element having an optically responsive composition is irradiated with a control light and a signal light having different wavelengths. The wavelength of the control light is selected from a wavelength band which is absorbed by the optically responsive composition. The optically responsive composition allows a thermal lens to be reversibly formed according to a distribution of density change caused by a temperature increase generated in and around an area of the optically responsive composition in which the control light is absorbed. The intensity and/or the luminous flux density of the signal light transmitted through the thermal lens can thereby be modulated. Japanese Patent Laid-Open Publication No. Hei 10-148853 further describes that the above-referenced optical element may be a dye/resin film or a dye solution film, and that a response time of the signal light in response to irradiation of the control light at a power of 2 to 25 mW is less than 2 microseconds.
The above-referenced thermal lens effect is explained as follows. At a center portion of light absorption, molecules which absorbed the light convert the light into heat. The heat is transferred toward the surrounding areas, thereby creating a temperature distribution. As a result, the refraction index of the light-transmitting medium changes from the center of light absorption toward the outward areas in a spherical manner, generating a distribution of refraction index which is small in the center of light absorption and increasing toward the outward areas. This produces a light refraction effect which functions as a concave lens. Such a thermal lens effect has already been employed for a considerable time in the field of spectral analysis, and, at the present, an ultra-high sensitivity spectral analysis for detecting light absorption by just one molecule is also possible (as described in Kitao FUJIHARA, Kei-ichiro FUWA, and Takayoshi KOBAYASHI, “Laser-Induced Thermal Lens Effect and Its Application to Colorimetry”, Chemistry, published by Kagakudojin, Vol. 36, No. 6, pages 432–438 (1981); and Takehiko KITAMORI and Shiro SAWADA, “Photothermal Conversion Spectral Analysis”, Analysis, published by The Japan Society for Analytical Chemistry, March 1994, pages 178–187).
As a method for deflecting an optical path using the thermal lens effect or a change in refraction index created by heat, Japanese Patent Laid-Open Publication No. Sho 60-14221 discloses deflecting light by supplying heat to a medium by a heat-generating resistor to produce a refraction index distribution. However, because this method involves generating heat with a heat-generating resistor and heating the medium via heat conduction, the problem of “heat diffusion” is integral to this method. In other words, heat diffusion hinders creation of a fine pattern of thermal gradient in a large area, making it difficult to obtain a desired refraction index distribution. Further, even when a photolithographic technique used for a semiconductor integrated circuit is employed to process the heat-generating resistor into a fine pattern, the actual results achieved have been limited, inevitably increasing the element size. When the element size is increased, the optical system also becomes larger and more complex. Moreover, because a heat-generating resistor is used to heat the medium via heat conduction, this method is integrally defective in that the response speed is slow and the frequency of change in refraction index cannot be increased.
Japanese Patent Laid-Open Publication No. Hei 11-194373 describes a deflection element comprising at least an optical element composed of an optically responsive composition, and intensity distribution adjusting means for irradiating light on the optical element in a light intensity distribution having a wedge-like shape. A refraction index distribution is created in the optical element using a control light, and the created refraction index distribution is used to deflect a signal light having a wavelength differing from that of the control light. Although this approach is advantageous in that light can be controlled by light, there exists a restriction that the deflection angle must be less than 30 degrees such that the switching direction of an optical path cannot be freely set.